Add tests for asynchronous generators

[Kentzo]

Asynchronous generators are defined by PEP 525 and supported since Python 3.6

Unfortunately the syntax change (yielding from a coroutine) is not backward compatible, previous versions of Python will raise a SyntaxError.

The test suite should be extended to allow version-specific tests. Since the error happens during interpretation, runtime checks are not sufficient. Therefore discovery mechanics should exclude the test module before an attempt to import it is made.

[Martiusweb]

There's already a solution for python 3.4 and async/await:

if sys.version_info >= (3, 5):
    from . import test_case_await as _using_await
else:
    _using_await = None

https://github.com/Martiusweb/asynctest/blob/master/test/test_case.py#L16

It works because tests are executed with

python -m unittest tests

which loads the "tests" package, importing the modules (except the one using async/await). These version-specific modules are only imported if the runtime can parse them.

Do you have a better solution in mind?

[Kentzo]

I'm using the py.test runner, yet to try this approach.